1. Field of the Invention
The present invention relates to an electroluminescence display device, and more particularly, to an organic electroluminescence display device and a fabricating method of the same.
2. Discussion of the Related Art
Recently, flat panel displays have been proposed as a display device due to their characteristics of being thin, light weight, and low in power consumption. The flat panel displays include a liquid crystal display, a plasma display panel, a field emission display, and an electroluminescence display.
Among these displays, the electroluminescence display may be categorized into an inorganic electroluminescence display device and an organic electroluminescence display device depending upon a source material for exciting carriers. The organic electroluminescence display (OELD) device has drawn a considerable attention due to its high brightness, low driving voltage, and natural color images from the entire visible light range. Additionally, the OELD device has a wide viewing angle and a great contrast ratio because of self-luminescence. Since the OELD device does not require an additional light source such as a backlight, the OELD device has small size and light weight and low power consumption as compared with the liquid crystal display. Furthermore, the OELD device can be driven by a low voltage of direct current (DC), and has a short response time of several microseconds. Since the OELD device is totally solid phase, it is sufficiently strong to withstand external impacts and has a greater operational temperature range. Additionally, the OELD device can be manufactured at a low cost. Particularly, only deposition and encapsulation apparatuses are necessary for manufacturing the organic EL devices, and thus a manufacturing process of the OELD device is very simple in contrast with the liquid crystal display or plasma display panel.
The organic electroluminescence display device may be classified into a passive matrix type and an active matrix type depending upon a driving method.
The passive matrix type, which does not have additional thin film transistors (TFTs), has been conventionally used. In the passive matrix OELD device, scanning lines and signal lines perpendicularly cross each other to be arranged in a matrix shape. Since a scanning voltage is sequentially applied to the scanning lines to operate each pixel, an instantaneous brightness of each pixel during a selection period should reach a value resulting from multiplying the average brightness by the number of the scanning lines to obtain a required average brightness. Accordingly, as the number of the scanning lines increases, applied voltage and current also increase. Therefore, the passive matrix OELD device is not adequate to a display of high resolution and large area because the device is easily deteriorated and the power consumption is high.
Since the passive matrix OELD device has many limitations in resolution, power consumption and lifetime, an active matrix OELD device has been researched and developed as a next generation display device requiring high resolution and large display area. In the active matrix OELD device, a thin film transistor (TFT) is disposed at each sub-pixel as a switching element for turning on/off each sub-pixel. A first electrode connected to the TFT is turned on/off by the sub-pixel and a second electrode facing the first electrode functions as a common electrode. Moreover, a voltage applied to the pixel is stored in a storage capacitor, thereby maintaining the voltage and driving the device until a voltage of next frame is applied, regardless of the number of the scanning lines. As a result, since an equivalent brightness is obtained with a low applied current, an active matrix OELD device of low power consumption, high resolution and large area may be made.
FIG. 1 shows a band diagram of a related art organic electroluminescence display. As shown in FIG. 1, the related art organic electroluminescence display includes an anode electrode 1, a cathode electrode 7, a hole transporting layer 3, an emissive layer 4, and an electron transporting layer 5 between the anode electrode 1 and the cathode electrode 7. The related art organic electroluminescence display device further includes a hole injection layer 2, which is disposed between the anode electrode 1 and the hole transporting layer 3, and an electron injection layer 6, which is disposed between the cathode electrode 7 and the electron transporting layer 5, to efficiently inject holes and electrons.
The holes and the electrons are injected into the emissive layer 4 through the hole injection layer 2 and the hole transporting layer 3 from the anode electrode and through the electron injection layer 7 and the electron transporting layer 5 from the cathode electrode 7, respectively, thereby generating an exciton 8 in the emissive layer 4. Then, light corresponding to energy between the hole and the electron is emitted from the exciton 8.
The anode electrode 1 is formed of a transparent conductive material having a relatively high work function such as indium-tin-oxide and indium-zinc-oxide. Light is observed at the anode electrode 1. On the other hand, the cathode electrode 7 is formed of an opaque conductive material having a relatively low work function, such as aluminum, calcium, and aluminum alloy.
In the OELD device, in order to display full color images, the organic emissive layers of red, green and blue are formed by sub-pixels, respectively, and an insulating material is used as a partition wall. The partition wall can separate adjacent organic emissive layers and adjacent cathode electrodes to be formed thereon by sub-pixels without a patterning process.
In the OELD device having the partition wall, a printing method is widely used by dropping an organic emissive material solution of an ink type at a portion between the adjacent partition walls.
FIGS. 2A to 2D illustrate a fabricating method of a related art organic electroluminescence display device using an inkjet method.
In FIG. 2A, an anode electrode 12 is formed on a substrate 10, which has an emission region “A” and a non-emission region “B” defined thereon. A buffer layer 14 is formed on the anode electrode 12 in the non-emission region “B”, and a partition wall 16 is formed on the buffer layer 14. The buffer layer 14 is widely made of silicon oxide (SiO2).
The partition wall 16 includes a polyimide and makes it possible that an organic emissive layer (not shown) and a cathode electrode (not shown) will be formed without a patterning process in the next step, wherein the organic emissive layer is formed by using an inkjet method. Although not shown in the figure, the partition wall 16 may have an inverse taper, that is, an inverse trapezoid having the top side longer than the bottom side depending on exposing extents.
In the inkjet printing method, a solution of an ink type is used, and the solution includes a water-soluble organic emissive material. Therefore, the anode electrode 12 and the buffer layer 14, which are disposed in the emission region “A”, should be hydrophilic in order to attach the solution thereto, while the partition wall 16, which is disposed in the non-emission region “B”, should be hydrophobic in order to prevent the partition wall 16 from being stained with the solution.
That is, in a high definition OELD fabricated by the inkjet printing method, before forming the organic emissive layer, a step to differ wettability of the elements in the emission region “A” from that in the non-emission region “B” is required.
In FIG. 2B, the substrate 10 including the partition wall 16 is disposed in a vacuum chamber (not shown), and a first plasma treatment using an oxygen (O2) gas is carried out to have the anode electrode 12 and the buffer layer 14 possess the characteristics of hydrophilicity. Next, a second plasma treatment using a carbon tetrafluoride (CF4) gas is carried out to have the partition wall 16 possess the characteristics of hydrophobicity. Therefore, the surface of the partition wall 16 has hydrophobicity and the surfaces of the anode electrode 12 and the buffer layer 14 exposed by the partition wall 16 have hydrophilicity.
In FIG. 2C, an organic electroluminescent layer 18, which may include a hole transporting layer, an organic emissive layer and an electron transporting layer, is formed on the substrates including the partition wall 16 of hydrophobicity by using an inkjet printing method. The organic electroluminescent layer 18 is made of a water-soluble material having an ink type. The organic electroluminescent layer 18 is formed on only the anode electrode 12 and the buffer layer 14 in the emission region “A,” and has a hemisphere shape because the anode electrode 12 and the buffer layer 14 are hydrophilic while the partition wall 16 is hydrophobic.
In FIG. 2D, a cathode electrode 20 is formed on the organic electroluminescent layer 18 by a deposition method using a metallic material of being reflective. A metal layer 19 made of the same material as the cathode electrode 20 is also formed on the partition wall 16. The metal layer 19 is disconnected from the cathode electrode 20. Therefore, the cathode electrode 20 is patterned without the patterning process because of the partition wall 16.
By the way, in the related art inkjet printing method, since plasma treatments are carried out two times, defects may be obtained in the unexpected area due to physical or chemical reaction from the plasma treatments. Additionally, a manufacturing process is complicated because of different plasma conditions depending on source materials.